Electrolytic capacitors are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. Typically, electrolytic capacitors have a larger capacitance per unit volume than certain other types of capacitors, making electrolytic capacitors valuable in relatively high-current and low-frequency electrical circuits. One type of capacitor that has been developed is a wet electrolytic capacitor that includes an anode, a cathode, and a liquid or “wet” electrolyte. Wet electrolytic capacitors tend to offer a good combination of high capacitance with low leakage current and a low dissipation factor. In certain situations, wet electrolytic capacitors may exhibit advantages over electrolytic capacitors in which the electrolyte is a solid. For example, wet electrolytic capacitors may, in certain situations, operate at a higher working voltage than solid electrolytic capacitors. Additionally, by way of example, wet electrolytic capacitors sometimes may be much larger in size than solid electrolytic capacitors, leading to larger capacitances for such large wet electrolytic capacitors.
In conventional wet electrolytic capacitors, the anode may be a metal foil, for example, a tantalum foil. The anode may also be a metal “slug,” for example, a “slug” of powdered tantalum material. As is known in the art, the term “slug” may refer to the anode body portion of a capacitor. A tantalum slug may be formed by mixing powdered tantalum particles with a suitable binder/lubricant to ensure that the particles will adhere to each other when pressed to form the anode. The powdered tantalum is compressed under high pressure around a tantalum wire and is sintered at high temperature under vacuum to form a sponge-like structure, which is very strong and dense but also highly porous. The porosity of the resulting tantalum slug results in the slug having a large internal surface area.
In certain wet electrolytic capacitors, the cathode is a container, which is filled with the liquid electrolyte. For example, the cathode may be a tantalum or tantalum-plated cylindrically-shaped container that acts as the negative terminal of the electrolytic capacitor. In these wet electrolytic capacitors, the liquid electrolyte and the porous, sintered anode are disposed within the cathode container. The wet electrolyte electrically connects the anode and the cathode, and thus, must have a certain conductivity. In many typical wet electrolytic capacitors, the liquid electrolyte is an aqueous solution of sulfuric acid.
Some conventional wet electrolytic capacitors are described in U.S. Pat. Nos. 5,369,547 and 6,594,140 to Evans, et al., both of which are incorporated herein in their entireties by reference thereto for all purposes. In the Evans, et al. '547 patent, a metal container having an inside surface and an outside surface functions as the cathode of the capacitor, and a porous coating is disposed at the inside surface of the container in electrical communication with the container. Similarly, in the Evans, et al. '140 patent, the cathode of a wet electrolytic capacitor includes a coating and is described as an electrochemical-type capacitor electrode. Additionally, in U.S. Pat. No. 6,721,170 to Evans, et al., packaged hybrid capacitors are described, and the cathode of such a hybrid capacitor is said to include a porous metal oxide film, preferably a metal oxide cathode layer of ruthenium.
Coatings, such as those described in the Evans, et al. patents mentioned above, may be applied to a metal substrate for use as a cathode in a wet electrolytic capacitor using various techniques, such as the methods described in the '547 and '140 Evans, et al. patents, as well as the substrate coating process disclosed in U.S. Pat. No. 6,224,985 to Shah, et al., which is incorporated herein in its entirety by reference thereto for all purposes.
Despite the development of various wet electrolytic capacitors with coated cathodes having certain improved properties, a need currently exists for improved wet electrolytic capacitors that exhibit increased effective cathode capacitance. Specifically, a need currently exists for wet electrolytic capacitors having an extremely large cathode capacitance, which enables such wet electrolytic capacitors to exhibit improved volumetric efficiency. The wet electrolytic capacitors of the present invention and the cathodes for use in such capacitors address these and other needs.